Introduction:
Respiratory status is one of the main factors affecting the length of survival in patients with Duchenne muscular dystrophy (DMD) – the most common, severe, progressive muscular dystrophy. Aim of the study was (1) to assess pulmonary function in DMD patients using the z-score method and (2) to identify factors affecting it, irrespective of disease progression.

Material and methods:
We evaluated 55 boys (aged 5–18 years) with DMD. The spirometry was performed with: forced vital capacity (FVC), forced expiratory volume in 1 second (FEV1), peak expiratory flow (PEF) analysis as absolute values (in litres or litres/min), % predicted value (%pv) and z-scores (z). Information on the need of ventilation support, ambulatory status and steroid therapy was collected.

Results:
25 (45%) subjects were non-ambulatory, 38 (69%) used steroid therapy. Mean FVC[z] –2.4 ±2.2, FEV1[z] –2.0 ±1.9, PEF[z] –1.5 ±1.3 values significantly decreased with age (r = –0.62/–0.65/–0.55; p < 0.001 respectively), after reaching the peak values in the range 9–12 or 6–9 years of age depending on the analysis method (absolute, %pv or z-score). The results fell below the normal range (z-score < –1.64) at the age of 9.8/10.4/11.6 years and below 80%pv at 10.7/12.2/13.2 for FVC/FEV₁/PEF, respectively. The pulmonary function test results were significantly lower in non-ambulant (p < 0.001) and non-steroid patients (p < 0.02).

Conclusions:
Analysis of the pulmonary function test based on z-scores shows that deterioration of pulmonary function in DMD males may occur earlier than we thought when measured by %pv and absolute values. Early loss of ambulation, lack of steroids and delayed steroid therapy are risk factors for worse pulmonary outcomes. To confirm these findings, cohort longitudinal studies are necessary.